feat: add clickable file paths in agent responses (web UI only)

[chenditc]

What does this PR do?

When an agent responds with inline code that looks like a file path (e.g., src/components/Button.tsx), clicking it now opens the file in the review panel.

Supported Path Formats:

• Relative: src/components/Button.tsx
• Dot-relative: ./src/foo.ts, ../bar.js
• Absolute: /data/opencode/packages/ui/src/file.tsx

If you paste a large clearly AI generated description here your PR may be IGNORED or CLOSED!

How did you verify your code works?

1. Started backend server: bun run src/index.ts serve --port 4567 in packages/opencode
2. Started frontend dev server: VITE_OPENCODE_SERVER_PORT=4567 bun run dev in packages/app
3. Opened web UI at http://localhost:3000
4. Created a session and asked agent to mention file paths
5. Verified:
   • Inline code shows pointer cursor on hover
   • Inline code shows underline on hover
   • Clicking a file path opens it in the review panel
   • Newly opened file tab is focused/active
   • File content loads correctly

Fixes #1168

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1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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